Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A conventional fuel cell consists of a fuel electrode (anode) and an oxidant electrode (cathode), separated by an ion-conducting electrolyte (electrolyte). The assembly of anode, cathode and electrolyte are referred to as a membrane electrode assembly. The electrodes are coupled electrically to a load (such as an electronic circuit) by electrical conductors. In the conductor, electrical current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H+) in acid electrolytes, or the hydroxyl ion (OH−) in alkaline electrolytes. Hydrogen is often used as the fuel for producing the electricity and can be processed from methanol, natural gas, petroleum, or stored as pure hydrogen. Direct methanol fuel cells (DMFCs) utilize methanol, in a gaseous or liquid form as the fuel, thus eliminating the need for reforming operations. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. At the fuel cell cathode the most common oxidant is gaseous oxygen, which is readily and economically available from the atmosphere for fuel cells used in terrestrial applications.
In liquid feed electrochemical fuel cells, one or more of the reactants is introduced to the electro catalyst in a liquid form. Examples of electrochemical fuel cells that can be operated with a liquid fuel feed are those employing a lower alcohol, most commonly methanol, as the fuel supplied to the anode (so-called liquid feed direct methanol fuel cells) and oxygen to the cathode. In fuel cells of this type the reaction at the anode produces protons, as in the hydrogen/oxygen fuel cell described above, however the protons (along with carbon dioxide) arise from the oxidation of methanol. An electro catalyst promotes the methanol oxidation at the anode. The methanol may alternatively be supplied to the anode as vapor, but it is generally advantageous to supply the methanol to the anode as a liquid, preferably as an aqueous solution, such as 2% methanol. In some situations, an acidic aqueous methanol solution is the preferred feed to the anode.
The anode and cathode reactions in a direct methanol fuel cell are shown in the following equations:Anode reaction: CH3OH+H2O→6H++CO2+6e−Cathode reaction: 3/2O2+6H++6e−.→3H2OOverall reaction: CH3OH+ 3/2O2→CO2+2H2OThe protons formed at the anode electro catalyst migrate through the ion-exchange membrane from the anode to the cathode, and at the cathode electro catalyst layer, the oxidant reacts with the protons to form water as a byproduct. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. As long as methanol and water and oxygen are fed to the fuel cell, the flow of electric current will be sustained by electron flow in the external circuit and ionic flow in the electrolyte.
In electrochemical fuel cells employing liquid or solid electrolytes and gaseous or liquid reactant streams, crossover of a reactant from one electrode to the other is generally undesirable. Reactant crossover may occur if the electrolyte is permeable to the reactant, that is, some of a reactant introduced at a first electrode of the fuel cell passes through the electrolyte to the second electrode, instead of reacting at the first electrode. Reactant crossover typically causes a decrease in both reactant utilization efficiency and fuel cell performance defined as the voltage output from the cell at a given current density or vice versa. For example, ion-exchange membranes typically employed in solid polymer electrochemical fuel cells are permeable to methanol and thus methanol that contacts the membrane prior to participating in the oxidation reaction can cross over to the cathode side.
Diffusion of methanol fuel from the anode to the cathode (fuel crossover) leads to a reduction in fuel utilization efficiency and to performance losses. Fuel utilization efficiency losses arise from methanol diffusion away from the anode because some of the methanol that would otherwise participate in the oxidation reaction at the anode and supply electrons to do work through the external circuit is lost. Methanol arriving at the cathode may be lost through vaporization into the oxidant stream, or may be oxidized at the cathode electro catalyst, consuming oxidant, as follows:CH3OH+ 3/2O2→CO2+2H2OThe oxidation of methanol at the cathode reduces the concentration of oxygen at the electro catalyst and may affect access of the oxidant to the electro catalyst (mass transport issues). Further, depending upon the nature of the cathode electro catalyst and the oxidant supply, the electro catalyst may be poisoned by methanol oxidation products, or sintered by the methanol oxidation reaction. Conventional methods for reducing crossover have involved mixing excess water with the fuel. However, excess water may lead to reduced fuel efficiency and higher costs associated with a water supply.